1. Field of the Invention
The present invention relates to a zoom lens and an imaging device. More specifically, this invention is suitable for a camera, such as a video or digital still camera, with imaging elements to receive light and relates to a zoom lens suitable for the correction of blurring and an imaging device equipped with the zoom lens.
2. Description of the Related Art
Already known is a method of forming an image of an object on the surfaces of imaging elements made of photoelectric-conversion elements, such as CCDs (Charge Coupled Devices) or CMOSs (Complementary Metal-Oxide Semiconductors), by using the imaging elements and converting the quantities of light of the image into electric outputs by using the photoelectric-conversion elements.
With the recent technological progress in the field of fine processing, the processing speed of central processing units (CPUs) and the degrees of integration of storage media have been increased and it has become possible to process a large quantity of image data at a high speed. Besides, the degrees of integration of light-receiving elements have been increased to make recording at higher spatial frequency possible and the sizes of light-receiving elements have been reduced to make it possible to produce more compact cameras.
Moreover, zoom lenses are in great demand to cope with various photo-taking situations, and zoom lenses of high zooming ratios in particular are in great demand.
However, the angle of view in the telephoto end state in an optical system of a high zoom ratio is small; therefore, the slight movement of the camera causes heavy blurring of the image. Known under the circumstances is a so-called electronic blurring-compensation system, especially for video cameras of high zoom ratios, which shifts the photo-taking area of the light-receiving elements to compensate for blurring.
Besides, already known is a blurring-compensation optical system wherein the deterioration of optical performance at the time of shift of the position of an image is prevented by shifting a group of some lenses of the lens system in a direction perpendicular to the optical axis of the lens system.
Such a blurring-compensation optical system may include a detecting unit to detect the movement of the camera due to the release of the shutter or the like, a control unit to give correction to the position of a group of lenses based on signals outputted from the detecting unit, and a driving unit to shift the group of lenses based on the output from the control unit.
With the above blurring-compensation optical system, the group of lenses is shifted by the driving unit and, thereby, an image is shifted and the blurring of the image due to the movement of the camera is compensated for.
Such blurring-compensation optical systems are disclosed in patent documents 1, 2, and 3.
In the case of the zoom lens of the patent document 1, a third lens group disposed on the image side of an aperture stop includes a negative and a positive subgroup and an image is shifted by shifting the positive subgroup.
In the case of the zoom lens of the patent document 2, a third lens group disposed on the image side of an aperture stop includes a positive and a negative subgroup and an image is shifted by shifting the positive subgroup.
In the case of the zoom lens of the patent document 3, an image is shifted by shifting the whole of a third lens group.    Patent document 1: Japanese Patent Laid-open No. 2002-244037    Patent document 2: Japanese Patent Laid-open No. 2003-228001    Patent document 3: Japanese Patent Laid-open No. 2003-295057
In the cases of the above blurring-compensation optical systems, a lens group in the vicinity of an aperture stop is shifted; therefore, a driving mechanism to shift the lens group, a mechanism to open and close the aperture stop, and a mechanism to move lenses along the light axis at the times of zooming and focusing are liable to interfere with one another. To avoid such interference, it is necessary to make the diameter of the lens-barrel large.
There is a need for solving the above problem and providing, without increasing the number of lenses, a compact blurring-compensation zoom lens and an imaging device equipped with the zoom lens.
According to an embodiment of the present invention, there is provided a zoom lens including a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, a fourth lens group with positive refractive power, and a fifth lens group including a negative subgroup with negative refractive power and a positive subgroup with positive refractive power. The first to fifth lens groups are arranged from the object side toward the image side in the order of the above description. When the state of lens positions changes from the state of the wide-angle end to the telephoto end state, (i) the first lens group is fixed with respect to a direction of an optical axis, (ii) the second lens group moves toward the image side, (iii) the third lens group is fixed with respect to the direction of the optical axis, (iv) the fourth lens group moves along the optical axis to compensate for the change of the position of the image surface caused by the movement of the second lens group, and (v) the fifth lens group is fixed with respect to the direction of the optical axis. The fourth lens group moves along the optical axis when the zoom lens is focused on an object at a short distance. An aperture stop is disposed in the vicinity of the third lens group. The lens surface of the negative subgroup of the fifth lens group on the image side and nearest to the image is concave, and the lens surface of the positive subgroup of the fifth lens group on the object side and nearest to the object is convex. The positive subgroup shifts in a direction approximately perpendicular to the optical axis to shift an image. The zoom lens satisfies the condition defined by the inequality of “0.2<f5p/ft<0.5”, where f5p is the focal length of the positive subgroup of the fifth lens group and ft is the focal length of the entire lens system in the telephoto end state.
According to an embodiment of the present invention, there is provided an imaging device including a zoom lens and an imaging element to convert an optical image formed by the zoom lens into electric signals. The zoom lens includes a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, a fourth lens group with positive refractive power, and a fifth lens group including a negative subgroup with negative refractive power and a positive subgroup with positive refractive power. The first to fifth lens groups are arranged from the object side toward the image side in the order of the above description. When the state of lens positions changes from the state of the wide-angle end to the telephoto end state, (i) the first lens group is fixed with respect to a direction of an optical axis, (ii) the second lens group moves toward the image side, (iii) the third lens group is fixed with respect to the direction of the optical axis, (iv) the fourth lens group moves along the optical axis to compensate for the change of the position of the image surface caused by the movement of the second lens group, and (v) the fifth lens group is fixed with respect to the direction of the optical axis. The fourth lens group moves along the optical axis when the zoom lens is focused on an object at a short distance. An aperture stop is disposed in the vicinity of the third lens group. The lens surface of the negative subgroup of the fifth lens group on the image side and nearest to the image is concave, and the lens surface of the positive subgroup of the fifth lens group on the object side and nearest to the object is convex. The positive subgroup shifts in a direction approximately perpendicular to the optical axis to shift an image. The zoom lens satisfies the condition defined by the inequality of “0.2<f5p/ft<0.5”, where f5p is the focal length of the positive subgroup of the fifth lens group and ft is the focal length of the entire lens system in the telephoto end state.
As described above, according to an embodiment of the present invention, an image is shifted by shifting a lens disposed away from an aperture stop in a direction approximately perpendicular to the optical axis.
The zoom lens according to an embodiment of the present invention includes a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, a fourth lens group with positive refractive power, and a fifth lens group including a negative subgroup with negative refractive power and a positive subgroup with positive refractive power. The first to fifth lens groups are arranged from the object side toward the image side in the order of the above description. When the state of lens positions changes from the state of the wide-angle end to the telephoto end state, (i) the first lens group is fixed with respect to the direction of the optical axis, (ii) the second lens group moves toward the image side, (iii) the third lens group is fixed with respect to the direction of the optical axis, (iv) the fourth lens group moves along the optical axis to compensate for the change of the position of the image surface caused by the movement of the second lens group, and (v) the fifth lens group is fixed with respect to the direction of the optical axis. The fourth lens group moves along the optical axis when the zoom lens is focused on an object at a short distance. An aperture stop is disposed in the vicinity of the third lens group. The lens surface of the negative subgroup of the fifth lens group on the image side and nearest to the image is concave, and the lens surface of the positive subgroup of the fifth lens group on the object side and nearest to the object is convex. The positive subgroup shifts in a direction approximately perpendicular to the optical axis to shift an image. The zoom lens satisfies the condition defined by the inequality of “0.2<f5p/ft<0.5”, where f5p is the focal length of the positive subgroup of the fifth lens group and ft is the focal length of the entire lens system in the telephoto end state.
The imaging device according to an embodiment of the present invention includes a zoom lens and an imaging element to convert an optical image formed by the zoom lens into electric signals. The zoom lens includes a first lens group with positive refractive power, a second lens group with negative refractive power, a third lens group with positive refractive power, a fourth lens group with positive refractive power, and a fifth lens group including a negative subgroup with negative refractive power and a positive subgroup with positive refractive power. The first to fifth lens groups are arranged from the object side toward the image side in the order of the above description. When the state of lens positions changes from the state of the wide-angle end to the telephoto end state, (i) the first lens group is fixed with respect to the direction of the optical axis, (ii) the second lens group moves toward the image side, (iii) the third lens group is fixed with respect to the direction of the optical axis, (iv) the fourth lens group moves along the optical axis to compensate for the change of the position of the image surface caused by the movement of the second lens group, and (v) the fifth lens group is fixed with respect to the direction of the optical axis. The fourth lens group moves along the optical axis when the zoom lens is focused on an object at a short distance. An aperture stop is disposed in the vicinity of the third lens group. The lens surface of the negative subgroup of the fifth lens group on the image side and nearest to the image is concave, and the lens surface of the positive subgroup of the fifth lens group on the object side and nearest to the object is convex. The positive subgroup shifts in a direction approximately perpendicular to the optical axis to shift an image. The zoom lens satisfies the condition defined by the inequality of “0.2<f5p/ft<0.5”, where f5p is the focal length of the positive subgroup of the fifth lens group and ft is the focal length of the entire lens system in the telephoto end state.
As described above, according to an embodiment of the present invention, the fifth lens group is disposed on the image side of the fourth lens group, and movable, in order to reduce the number of lenses and the weight of the fourth lens group and simplify the mechanism for moving the fourth lens group. Thus, the lens system is made compact.
If the zoom lens according to an embodiment of the present invention is to be equipped with a blurring-compensation mechanism, the mechanism for shifting the lens in a direction approximately perpendicular to the optical axis does not interfere with the lens-driving mechanism for zooming and focusing or the mechanism for opening and closing the aperture stop. Accordingly, the lens barrel, and its diameter in particular, can be made small. In addition, by satisfying the condition defined by the inequality of “0.2<f5p/ft<0.5”, coma aberration is well compensated for and the lens-shifting mechanism for compensating for blurring is made compact.
The imaging device according to an embodiment of the present invention including the above zoom lens is compact if it is equipped with a blurring-compensation mechanism, and it produces images wherein coma aberration is well compensated for.
According to embodiments of the present invention, the zoom lens satisfies the condition defined by the inequality of “0.4<rp/Df<0.8”, where Df is the distance along the optical axis from the aperture stop to the lens surface of the positive subgroup of the fifth lens group on the object side and nearest to the object and rp is the radius of curvature of the lens surface of the positive subgroup of the fifth lens group on the object side and nearest to the object. Accordingly, the sensitivity to the inclination of the optical axis between the negative and positive subgroups is reduced and, hence, the effect of assembling precision on the performance is reduced. Besides, the variation of coma aberration in the off-axis area of the image plane, which occurs when blurring is compensated for, is reduced.
According to embodiments of the present invention, the zoom lens satisfies the condition defined by the inequality, “−0.2<(rp−rn)/(rp+rn)<0.2”, where rn is the radius of curvature of the lens surface of the negative subgroup on the image side and nearest to the image. Accordingly, the variation of coma aberration, which occurs in the off-axis area of the image plane when blurring is compensated for, is better compensated for.
According to embodiments of the present invention, the zoom lens satisfies the condition defined by the inequality of “−0.2<ft/f5<0.2”, where f5 is the focal length of the fifth lens group. Accordingly, the zoom lens is made further compact.
According to embodiments of the present invention, the positive subgroup comprises a positive lens and a negative one, the latter disposed on the image side of the former. Accordingly, the variation of coma aberration, which occurs when blurring is compensated for, is reduced and the overall length of the zoom lens is further reduced.